Digital cameras sample the original light that bounces off a subject to create a digital image. A digital camera may use a shutter in combination with a sensor to determine an exposure time and acquire the proper amount of light to make a good image. The proper amount of light may come from accumulating ambient light over time until the proper amount of light is obtained. Alternatively, the addition of artificial light from a flash speeds up the amount of time until the proper amount of light is obtained to make a good image.
In a typical mechanical shutter, the timing between scan lines of a sensor and the actuation of the shutter is such that the shutter would be closed until the sensor, such as a complementary metal-oxide semiconductor (CMOS) sensor or a Charge Coupled Device (CCD) sensor, was ready to be exposed to the light in the picture frame. When the mechanical shutter opens it goes to a very discrete second distinct state. In the second distinct state, nearly 100% of the light is allowed to pass and be sensed by light sensitive pixels in the sensor. When the mechanical shutter gives the command to close, and the shutter closes and then the light no longer reaches the sensor.
CMOS sensors with a rolling shutter may have a relatively slow exposure time when a flash is required. Typically, the exposure time may be greater than 1/10th of a second. Often, in a scene having ambient light, the scene may be exposed for ⅛th of a second, which means a hand-held camera may yield a blurry photograph. In a sensor implementing a rolling shutter, the light accumulating pixels in the sensor both track the accumulation of charge and are read out in a pixel line by pixel line manner. Since the integration process moves through the image over some length of time, some motion artifacts may become apparent. For example, if a vehicle is moving through the image during capture, then light from the top of vehicle will be integrated at some earlier time than light from the bottom of the vehicle, causing the bottom of the vehicle to appear slanted forward in the direction of motion.
Some digital cameras have a mechanical shutter mechanism to control the exposure time of the CMOS sensor. Unfortunately, the inclusion of mechanical shutter adds significant expense to the cost of a camera. Further, a mechanical shutter due to its size and dimensions may add physical bulkiness to the camera.
Additionally, in some digital cameras, after a flash flashes, the ambient light reflecting off the objects in the picture frame while the mechanical shutter is open may cause problems due to the movement of those objects. For example, if the photoflash provides 75% of the exposure light and the ambient light provides the remaining 25% over the entire integration time, then the light from ambient source can cause a ghost image or blurring on the resultant digital image due to the movement of the objects.
Some cameras may have sensors implementing a global shutter timing mechanism for the light sensitive pixels. These sensors have electronic components added to each pixel location in the sensor. Typically, in a sensor implementing a global shutter timing mechanism all the pixels begin the integration time at the same time, integrate over the same interval of time, and at each pixel are simultaneously transferred to a light insensitive storage area at the same time. Therefore, sensors employing a global shutter timing mechanism typical have little to no potential for motion artifacts.